Tuesday 14 November 2023

Creationism in Crisis - Green Algae Show How Multicellular Life Evolved

Chlamydomonas, an alga existing on the border between a single-celled and multicellular organism.
New discovery on how green algae count cell divisions illuminates key step needed for the evolution of multicellular life

One of the most important developments in the evolution of life on Earth was the evolution of multicellularity from single-celled organisms, but all multicellular organisms start out as a single cell then needs to divide multiple times and the resulting cells need to differentiate into the different specialties to gain the benefits of multicellularity. The question is, how did this process evolve from a simple cluster of identical cells, each just a copy of the 'mother'cell, to a controlled, organised multicellular organism?

One of the species hovering on the border between a single-celled existence and multicellularity is the green alga, Chlamydomonas, a eukaryote organism with relatives that have evolved into multicellular organisms multiple times.

Now, an international research team led by James Umen, PhD, of the Donald Danforth Plant Science Center, St Louis, MO, USA, has made an unexpected discovery that sheds some light on how the initial stages of forming a colony of daughter cells in Chlamydomonas is regulated.

First, a little AI background to Chlamydomonas:
What information is there about Chlamydomonas? Chlamydomonas is a genus of green algae that belongs to the Chlorophyta division. These single-celled organisms are known for their simple structure and ability to perform photosynthesis. Here are some key points about Chlamydomonas:
  1. Cell Structure: Chlamydomonas cells are typically biflagellate, meaning they have two flagella that help them move through water. The cells are generally ovoid or spherical in shape and contain a single cup-shaped chloroplast, a nucleus, a pyrenoid (a proteinaceous structure involved in starch storage), and a contractile vacuole for osmoregulation.
  2. Habitat: Chlamydomonas species are found in various aquatic environments, including freshwater, soil, and even snow. They are often part of the phytoplankton community.
  3. Reproduction: Chlamydomonas reproduces both sexually and asexually. Asexual reproduction occurs through cell division (binary fission). Sexual reproduction involves the fusion of specialized haploid gametes (isogametes) to form a diploid zygote. Under certain conditions, the cells can also produce dormant structures called zygospores.
  4. Photosynthesis: Like other green algae and plants, Chlamydomonas is capable of photosynthesis, a process by which light energy is converted into chemical energy. Chlorophyll, along with other pigments, is responsible for capturing light during photosynthesis.
  5. Research Model: Chlamydomonas has been extensively studied as a model organism in various scientific fields, including cell biology, genetics, and biochemistry. Its simple structure and ease of cultivation make it a useful organism for research.
  6. Genome: The genome of Chlamydomonas reinhardtii, in particular, has been sequenced, providing valuable information for genetic and molecular studies. This has contributed to our understanding of various cellular processes, including photosynthesis and flagellar movement.
  7. Biotechnological Applications: Chlamydomonas has potential applications in biotechnology, including the production of biofuels. Researchers are exploring ways to engineer Chlamydomonas to enhance its ability to produce biofuel precursors.
  8. Environmental Indicators: The presence and abundance of Chlamydomonas in aquatic ecosystems can serve as indicators of environmental conditions, including nutrient levels and water quality.
In summary, Chlamydomonas is a versatile organism that has contributed significantly to our understanding of fundamental biological processes. Its simplicity and amenability to laboratory conditions make it a valuable model organism in scientific research.

Can you explain the relationship between cell volume and cell surface area, and why this limits cell size, please?

The relationship between cell volume and cell surface area is crucial for understanding the limitations on cell size. This concept is often described by the surface area-to-volume ratio. As a cell grows larger, its volume increases more rapidly than its surface area, leading to changes in the surface area-to-volume ratio. This has several important implications for cell biology:
  1. Surface Area-to-Volume Ratio:
    • The surface area-to-volume ratio is a critical factor influencing cell size. It is calculated by dividing the cell's surface area by its volume.
    • The formula is ( Surface Area Volume )
  2. Surface Area Importance:
    • The cell membrane is responsible for exchanging materials with the external environment. A larger surface area allows for more efficient exchange of nutrients, gases, and waste products.
    • Nutrients and oxygen must diffuse across the cell membrane to reach the cell's interior, and waste products must diffuse out. A larger surface area facilitates these processes.
  3. Volume Importance:
    • The cell's volume represents its internal content, including organelles and the cytoplasm. As a cell grows, its volume increases, requiring more resources and energy to maintain cellular activities.
  4. Surface Area-to-Volume Ratio Decrease:
    • As a cell enlarges, its volume increases more rapidly than its surface area. This results in a decrease in the surface area-to-volume ratio.
    • A decreasing ratio means that, relative to its size, the cell has less membrane area available for nutrient exchange per unit of internal volume.
  5. Limitations on Cell Size:
    • Cells need to maintain an adequate surface area-to-volume ratio to support essential cellular functions. If a cell becomes too large, the ratio becomes insufficient for efficient nutrient exchange and waste removal.
    • Large cells may struggle to meet the demands of their increased volume, leading to decreased efficiency in cellular processes.
  6. Microscopic and Macroscopic Implications:
    • This principle applies not only to microscopic cells but also to larger organisms. The size and shape of cells in multicellular organisms are influenced by the need to maintain an appropriate surface area-to-volume ratio.
In summary, the surface area-to-volume ratio is a critical factor that limits cell size. Cells must balance the need for a large enough surface area to facilitate nutrient exchange with the need to maintain a manageable volume. This balance is essential for the efficient functioning of cells and, by extension, the organisms they comprise.
The reason a growing single-celled organism divides in the first place is because at it grows the ratio of the surface are to volume decreases exponentially, so, as the demand for nutrients and the need to get rid of waste increases, the ability to do so through a relatively shrinking surface area decreases. It is therefore a simple mathematical exercise to calculate when a cell would be expected to divide to restore an optimal surface area to volume ratio in the daughter cells. The following chart should make this clear:
Chart showing the relationship between volume and surface area in a cell. The green line shows how the ratio of surface area to volume decreases rapidly as the cell grows, quickly depriving the cell of the means to exchange nutrients and gasses with its environment.
However, the team noticed that Chlamydomonas cells don't conform to the predictions. They either don't divide at all, or they divide multiple times. The way they control this mechanism could have implications for the control of certain cancers in humans that arise because a cancer cell loses the ability to regulate how many times it divides.

As the Donld Danforth Plant Science Centre news release explains:
New discovery on how green algae count cell divisions illuminates key step needed for the evolution of multicellular life

An international research team led by James Umen, PhD, member, Donald Danforth Plant Science Center has made an unexpected discovery of a biased counting mechanism used by the single-celled green alga Chlamydomonas to control cell division. Chlamydomonas cells can grow very large and then divide multiple times in succession. The team found that the number of divisions a mother cell undertakes to restore its daughters to the correct starting size deviates from the mathematical optimum that was assumed to dictate this process. Instead, mother cells almost never divided just one time—they either didn’t divide at all or divided two or more times. This unexpected bias against a single division has important implications for understanding the evolution of multicellular life and provides a new avenue for engineering algal cells for improved yields of biofuel and high value products. The article describing these findings titled, "A Cell-Based Model for Size Control in the Multiple Fission Alga Chlamydomonas reinhardtii," was published on November 9, 2023, in the journal Current Biology.

Chlamydomonas cells, like those of many other algae and single-celled protists, can grow very large before they divide. This atypical growth and division pattern lets them make optimal use of light and nutrients, but also creates a problem in size control: Under some conditions cells will just barely double their size before it is time to divide and only need to divide once; but under favorable conditions the same cell might grow more than ten times its starting size and would need to divide multiple times in succession to produce daughters of the correct size. This size variability presents a conundrum which was solved by the evolution of a mechanism in Chlamydomonas that enables cells to assess their size and count out the correct number of cell divisions.

It was always assumed that the division pattern was dictated by a simple relationship between mother cell size and number of divisions, and models that assume this simple relationship can accurately predict the behaviors of entire cell populations, but by looking at division behaviors of thousands of individual cells of varying sizes we found an unanticipated dearth of cells dividing just once.

The bias against one division we observed in Chlamydomonas was very likely present in direct ancestors of its multicellular relatives and was further amplified as they evolved greater size and complexity.

Dr. James Umen, corresponding author
Donald Danforth Plant Science Center, St. Louis, MO, USA.
Instead, cells that should have divided once opted to not divide at all, and most cells only became able to divide when they had more than doubled in size.

Faced with this unexpected result, team members Abhyudai Singh, PhD, professor, University of Delaware and César Augusto Vargas-García, PhD, analytics team leader, AGROSAVIA -Corporación colombiana de investigación agropecuaria, Bogotá, Colombia, used mathematical modeling to come up with a more accurate predictive model for the behavior of the cells, while the research team at Danforth Center, spearheaded by Dianyi Liu, PhD, postdoctoral associate, dug deeper to understand the genetic mechanisms that produced the observed counting bias. The team discovered that a well-known, but still poorly-understood genetic mechanism for controlling cell division found in algae, plants and humans - called the retinoblastoma tumor suppressor pathway - plays a critical role in preventing just one division.

While we are just at the start of understanding how the retinoblastoma pathway works in algae, the discovery of a mechanism for introducing bias in cell division behavior immediately suggests a way that cells modified their division behavior as an important step in the evolution of multicellular life.

Dr. Dianyi Liu, first author
Donald Danforth Plant Science Center, St. Louis, MO, USA
And Department of Biology
University of Missouri - St. Louis, St. Louis, MO, USA.
Multicellular relatives of Chlamydomonas not only skip the option to undergo one division but can prevent division until they have grown many times in size. This enables a single cell to rapidly produce a whole new multicellular individual with hundreds or even thousands of cells, an ability that is critical for fitness and survival.

While it remains unclear why the cells evolved a bias against dividing just once, the knowledge of this mechanism and its genetic control has practical implications in algal biotechnology where cell size can impact yields of high value products and even susceptibility to predation of algae by filter feeders in open pond cultures.

Looking forward, the team is now focused on understanding and modeling the specific mechanisms used by the retinoblastoma pathway to alter cell division behavior in algae, work that may lead to advances in algal biotechnology and even shed light on how the retinoblastoma pathway keeps human cells from developing cancer and prevents plant cells from dividing at the wrong time and place.
More technical detail is given in the team's paper in Current Biology:

Understanding how population-size homeostasis emerges from stochastic individual cell behaviors remains a challenge in biology.1,2,3,4,5,6,7 The unicellular green alga Chlamydomonas reinhardtii (Chlamydomonas) proliferates using a multiple fission cell cycle, where a prolonged G1 phase is followed by n rounds of alternating division cycles (S/M) to produce 2n daughters. A “Commitment” sizer in mid-G1 phase ensures sufficient cell growth before completing the cell cycle. A mitotic sizer couples mother-cell size to division number (n) such that daughter size distributions are uniform regardless of mother size distributions. Although daughter size distributions were highly robust to altered growth conditions, ∼40% of daughter cells fell outside of the 2-fold range expected from a “perfect” multiple fission sizer.7,8 A simple intuitive power law model with stochastic noise failed to reproduce individual division behaviors of tracked single cells. Through additional iterative modeling, we identified an alternative modified threshold (MT) model, where cells need to cross a threshold greater than 2-fold their median starting size to become division-competent (i.e., Committed), after which their behaviors followed a power law model. The Commitment versus mitotic size threshold uncoupling in the MT model was likely a key pre-adaptation in the evolution of volvocine algal multicellularity. A similar experimental approach was used in size mutants mat3/rbr and dp1 that are, respectively, missing repressor or activator subunits of the retinoblastoma tumor suppressor complex (RBC). Both mutants showed altered relationships between Commitment and mitotic sizer, suggesting that RBC functions to decouple the two sizers.
Graphical abstract

It must be galling for creationists to have to keep pretending to themselves that there is no scientific evidence for evolution or rational explanation for how complex multicellular organisms evolved without intelligent [sic] intervention, only to have scientists producing the evidence they claim isn't there, and homing in on the natural mechanisms for it. But then intellectual dishonesty and the ability for willful self-delusion are basic membership qualifications for the creationist cult.

Thank you for sharing!

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